1. Field of the Invention
This invention relates to multiple point attachment to specimens, for biaxial or uniaxial loading, deformation or testing.
Situations exist where it desirable to induce specified loads, deformations or strain states in materials, specimens or objects or to restrain them against motions that would otherwise occur. These include situations where mechanical properties are determined by applying particular loads or combinations of loads in different directions or loading in one direction with or without restraint in another. Other situations include, but are not limited to, manufacturing processes, pre-conditioning of materials, use of stress or strain states to induce fiber alignment, crystallization in the material or situations where it is desired to otherwise regulate, alter or transform the material's properties or structure at the bulk, meso, micro or nano scale.
Attachment to the specimen is often of concern and, to produce specified internal strain or stress fields, careful design of the specimen and its loading system may be required, especially if uniform or other specified fields are desired or if large deformations are involved.
2. Description of the Prior Art
There are several methods known in the art used to load materials. One of the key difficulties is attachment of the load system to the specimen.
The three primary methods can be characterized as either structured specimen methods, substrate methods or attachment methods.
Structured Specimen Methods
Structured specimen methods are characterized as structuring the material into a geometry that facilitates loading and deformation control. A well-known version of this is the pressurized cylinder test, in which the material of interest either occurs as a cylindrical shape or is formed into one. The ends of the specimen are clamped to circular end plates. By controlling the pressure inside the hollow specimen and controlling the spacing between the end plates, through load or deformation, the stresses and strains in the hoop and axial directions can be controlled independently. Primary loading is in the local tangent plane to the specimen; i.e., the loading is “in-plane”.
This method has the advantage of allowing the in-plane conditions to be specified independently (subject to certain limitations). It works well for testing of tubes or blood vessels.
Substrate Methods
Substrate methods are characterized as affixing the material to be tested onto a substrate material. The substrate is then loaded by stretching it in a known manner, thus also stretching the material under test.
For example, Love et al. (U.S. Pat. No. 6,833,924 and others) teaches a method where the substrate is secured along its edges and is pressurized to form a dome having a height axis substantially perpendicular to its original plane.
In general with dome substrate methods, the induced stress or strain states are not uniform over the surface of the dome. This method has the advantage that it can perform multi-axial loading since the material is stretched biaxially. Subtracting the effects of the substrate is a source of error in the system.
Attachment Methods
The third method can be characterized as attachment methods. The invention is related to this classification of test method. The attachment method is generally characterized as taking a small section of the material (a specimen) to be tested and attaching to it in such a manner that the edge load or deformation can be specified in one or more axes. Attachment of the load to the material is known to be problematic in the art. Gripping, clamping, hooks and suture attachments are known. Substantially different experimental results can be obtained using different gripping methods on the very same specimen. Sun et al. (Journal of Biomechanical Engineering, August 2005, Vol. 127/709) teaches the importance of attachment geometry in planar biaxial testing. They conclude that suture based methods are a preferred attachment method for biaxial mechanical testing of biological materials.
Gripping or Clamping Methods.
One approach is to cut the specimen into a square and clamp along each edge as shown in FIG. 1a. 
Another approach is cut the specimen into an “X” or “t” shape (also known as a cruciform method) and clamp on the arms as shown in FIG. 1b. 
Attachment Point Methods
Still another approach is to have a number of attachment points along the edges of the specimen. For biaxial or uniaxial testing the attachments must be stiff in the direction in which they are pulling or pushing and flexible in the other in-plane direction. Sutures are attached through the specimen along the edge of the specimen, and such sutures are considered state of the art. They simultaneously satisfy the stiffness and flexibility requirements. Sutures are shown in FIG. 1c. Hooks attached to sutures have also been used as shown in FIG. 1d. This method reduces the artificial stiffening at the boundaries.